An analog buffer is coupled between the source of an input voltage and a load when the source of the input voltage is unable to directly drive an output load. The analog voltage buffer has high input impedance and low output impedance to provide a source and a sink current and supplies an output voltage which tracks the input voltage. A linear transconductor is typically used in an analog voltage buffer to perform linear voltage-to-current conversion. Linear transconductors are typically used in continuous-time filters, tuning circuitry, and phase locked loops.
FIG. 1 is a circuit diagram of a prior art differential-mode analog buffer 100. A differential input voltage Vi (Vip-Vin) is applied to the respective non-inverting inputs of servo amplifiers 102, 103. The analog buffer 100 converts the differential input voltage Vi (Vip-Vin) to an output current io that flows through the output terminals of the analog buffer through the load. The analog buffer 100 includes four constant current sources labeled I1, I2, I3, I4 each constant current source is represented by a symbol having an arrow indicating the direction of the current flow on the input side of the current mirror.
Each of the servo amplifiers 102, 104 has an NPN bipolar transistor in the feedback path between the inverting input and the output. The base of NPN bipolar transistor Q1 in the feedback path of servo amplifier 102 is coupled to the output of the servo amplifier 102 and the emitter is coupled to the non-inverting input of the servo amplifier 102. Constant current source I1 is coupled to the collector of NPN transistor Q1 and constant current source I3 is coupled to the emitter of NPN transistor Q1.
The base of transistor Q2 in the feedback path of servo amplifier 104 is coupled to the output of the servo amplifier 104 and the emitter is coupled to the non-inverting input of the servo amplifier 104. Constant current source I2 is coupled to the collector and constant current source I4 is coupled to the emitter. A linear resistor RE is coupled between the emitter of transistor Q1 and the emitter of transistor Q2. The load RL coupled to the differential output Vo (Vop-Vom) of the analog buffer 100 is represented by load resistors RL/2 each coupled between the respective collector of the transistor Q1, Q2 and a common mode output voltage Vcmout.
A differential analog input voltage Vi (Vip-Vim) is applied to the analog buffer 100 through the respective non-inverting inputs of the servo amplifiers 102, 104. The corresponding differential output voltage Vo (Vop-Vom) on the collectors of the respective NPN transistors Q1, Q2 tracks the differential input voltage Vi (Vip-Vim)
If each of the constant current sources I1, I2, I3, I4 supplies a constant current through the range of operation of the circuit, the differential input signal Vi (Vip-Vim) is essentially equal to the voltage V (Vp-Vm) applied across the linear resistor RE. Thus, the voltage V across RE more or less follows the differential input voltage Vi (Vip-Vim) dependent on the gain-bandwidth of the feedback loops constituted by the combination of the servo amplifiers 102, 104 and the transistors Q1, Q2. Hence, the resulting output current io=Vi/RE flows through the output terminals. The same signal appears on Vop, Vom, but it is 180 degrees out of phase with the signal on the emitter; that is, Vp, Vm.
However, the output voltage Vop, Vom swing is limited. The lower output swing of Vop, Vom is limited because transistors Q2 and Q1 must always be in the active region to provide the feedback path for the respective servo amplifier so that the feedback loop is closed. In the active region, the base-emitter diodes in transistors Q2 and Q1 are forward biased and the collector-emitter diodes are reverse biased. Thus, with both transistors Q1, Q2 in the active region, there is a common-mode direct current (DC) voltage at Vom, Vop that is at least Vbe above ground because of the need to keep at least a Vbe voltage at Vm, Vp to keep transistors Q2 and Q1 forward biased. With low power supply voltages, for example, 2V, a common-mode D.C. voltage of 0.8V is significant.
FIG. 2 illustrates the lower output swing limitation of the prior art analog buffer shown in FIG. 1. The common mode voltage at VB1 is at least Vbe above the common mode voltage at Vip to keep transistor Q2 in the active region. Thus, the lower output swing of the output signal Vom is limited because the output voltage Vom must be greater than the voltage at VB1 to keep transistor Q2 in the active region.
Returning to FIG. 1, the upper output swing of Vop, Vom is limited because collector currents I2 and I1 must be kept constant. The requirement to keep the collector currents constant is dependent on the implementation of the constant current sources I2, I1, but it generally requires a voltage drop across the constant current sources. Thus, the power supply voltage Vdd limits the upper output swing and the feedback path through transistors Q1, Q2 limits the lower output swing. The limitations on the upper and lower output swing results in harmonic distortion of the output signal.
An analog buffer which offers low harmonic distortion for an output signal with a wide voltage swing and low power supply voltage is presented. The output voltage swing is increased by adding a voltage level shifter to the feedback path of a servo amplifier. The servo amplifier receives an input signal at a first input. A bipolar transistor is coupled to the output of the servo amplifier. The emitter of the bipolar transmitter is coupled through a feedback loop to a second input of the servo amplifier. A voltage level shifter is coupled in the feedback loop and a current source pushes current into the voltage level shifter to cause a voltage shift at the emitter of the bipolar transistor to increase output voltage swing. The voltage shift increases the lower voltage swing. The upper output voltage swing may be increased, by coupling a load between the collector of the bipolar transistor and the upper power supply voltage. The current source maintains a constant voltage at the second input of the servo amplifier through the diode to keep the bipolar transistor in an active region.
The voltage level shifter may be a diode-coupled NPN or PNP transistor or a Vbe multiplier. The bipolar transistor may be NPN or PNP. The diode may be a diode-coupled NPN bipolar transistor. The current source may be a cascode current mirror. A second current source may be coupled to the collector of the bipolar transistor to center the output voltage swing in the middle of the power supply voltage range.